Fiber optic lasers naturally oscillate in both available electromagnetic radiation polarizations. However, single polarization laser output is required in a large proportion of laser systems. Components that use polarized electromagnetic radiation include heterodyne detectors, most modulators, frequency doublers and shifters, and many amplifiers. Thus, the ability to generate single polarization fiber laser output is critical to most applications for fiber laser technology, including but certainly not limited to laser communications, laser radar for military and automotive applications, and active imaging systems. However, presently, no suitable method exists for polarizing fiber laser output that does not incur splicing losses between dissimilar fibers or require high-risk, labor-intensive fabrication steps
A number of techniques have been proposed and/or demonstrated for making a fiber laser oscillate in only one polarization. One such method requires splicing single polarization fiber onto the doped fiber. However, presently available single polarization fibers do not match the parameters of the doped fibers used for the laser gain; numerical aperture and core diameter, for example, are considerably different. Thus, the internal splice between the dissimilar fibers creates undesired loss.
In another technique the doped fiber is polished to its core and then metallic or waveguided loss of one of the polarizations is introduced into the fiber. However, this approach is labor-intensive and not cost-effective, and such techniques present a high risk of breaking the fiber laser.
Another proposed technique, though never successfully demonstrated, is to use the natural birefringence of a fiber grating by using two fiber segments spliced back together after rotating one segment by 90°. However, this proposed technique is labor-intensive, is not cost-effective, and presents a high risk of fiber breakage.